The present invention relates to the control of a rotary crop shear in a hot strip mill.
In a hot strip mill, the roughing stands reduce hot slabs to a thickness and width appropriate to finishing mill requirements. This results in a bar, the ends of which are deformed from the ideal shape, for presentation to the input of the finishing mill. When viewed in plan, the front ends may be pointed or rounded, while the tail end is scalloped or fish tailed.
In order to present bars correctly for finishing, some form of crop shear is employed to trim the heads and tails to approximately square plan. This results in a loss of yield and it is desirable with a view to minimizing costs and wastage to crop at the optimum point.
Failure to present a square head end results in poor threading of the finishing mill with associated high risk of cobbles. Equally, difficulties on the run out table and at the coiler occur due to the very pointed shapes which easily catch and lead later to cobbles. At the tail end, the expanding fish tail develops into long fingers which brake off and damage finishing rollers. In some cases the ends broken off are left in the mill endangering subsequent threading.
According to standard mill practice, the head may either be cut with the bar stationary or on the fly. Clearly, static crop is easier to control since one can visually decide upon a minimum loss crop point, say where the bar width has reached a high proportion of average width. Under operator control, the bar is progressively presented to the crop shear until the operator is satisfied with the cut.
Where the practice is to cut on the fly, the operation is much less accurate as the operator has to predict the arrival of the desired bar cutting point at the crop shear which is itself subject to delay in starting up.
The tail end cut, on the other hand, must always be effected on the fly because the head of the bar is now in the finishing mill and will probably be at the coiler so that the bar speed at the tail end is controlled by the input stand of the finishing mill. To be safe, mill operators have tried to cut off more tail than is required rather than suffer the costs of cobbles and roll wear, which result in mill down times.
There has been previously proposed in British Patent Specification No. 1,575,901 to monitor the progress of the bar to determine its speed at a predetermined trigger point and to commence the operation of the flying crop shear such as to cut off a predetermined length from the head or tail of the bar.
This system has the disadvantage that the amount cut off is always fixed whereas the optimum amount to be cropped is not constant but varies from one bar to the next. Thus, in deciding upon a predetermined length to cut off, it is necessary to allow for the worst case, and this often results in a substantial wastage.